1. Field of the Invention
This invention relates to an apparatus for handling a continuous plastic web and forming three-dimensional articles therefrom, and more particularly, a thermoforming apparatus wherein the web is advanced through a forming apparatus including means for heating the web, a forming press capable of forming a plurality of articles from the heated web by pressure differential and matched die forming techniques on a continuous basis, and means for advancing the formed articles and surrounding web matrix out of the press in such a manner so as to more precisely index the web and move it faster without tearing or distorting it than had previously been achieved heretofore in such type of apparatus.
2. Description of the Prior Art
Thermoforming apparatus capable of forming thousands of plastic articles by pressure differential and match die forming techniques on a continuous basis are well-known. An example of such forming equipment is illustrated in U.S. Pat. No. 3,113,345, issued Dec. 10, 1967 to Butzko. In this device, a plastic sheet is advanced between a pair of relatively movable platens of a forming press. Heating means on one of the platens softens the web and positive fluid pressure, such as provided by forced air, is exerted through one of the platens to blow the web against the heated area of the other platen. Once the web has been softened, the air flow is reversed and the heated portion of the web is drawn by a vacuum into a mold cavity in the unheated platen and conforms closely to the walls thereof. Positive fluid pressure, such as provided by forced air, can be exerted through the opposite, heated platen, to aid in forming the web and conforming it to the mold cavity in the opposite platen. The relatively cold surface of the mold cavity cools the formed article.
Cutting means such as a peripheral knife is also provided on one of the platens. Initially, the knife clamps the web to the heated platen when the platens move downwardly relative to each other. Once the softened web is formed by drawing it into one of the mold cavities by the pressure differential technique referred to above, slight movement is given to the knife bearing platen to completely extend it and have it either cut completely through or score the periphery of the formed article in the web matrix.
Upon opening of the platens relative to each other, the formed portions of the web are advanced to a position wherein the formed articles can be stripped from the web, and a subsequent portion of the web disposed in the press. This may be accomplished by bending the edges of the web matrix from a linearly advancing path while interposing an obstruction in the path of the web beneath the formed article to strip it therefrom. Alternatively, the scored or cut articles may be manually removed from the web as it is advanced. An example of such stripping mechanism is illustrated in U.S. Pat. No. 3,166,790, issued to Keyes on Jan. 26, 1965.
Most prior art thermoforming apparatus move or advance the plastic web sheet with a chain mechanism bearing pins which pass through the opposed edges of the plastic sheet. An example of such a structure is illustrated in U.S. Pat. No. 3,348,748, dated Oct. 24, 1967 issued to O'Brien or U.S. Pat. No. 3,785,762, issued Jan. 15, 1974 to Butzko. These chain mechanisms, when driven relatively fast to provide quick indexing and high volume production of the formed articles tend to overrun due to the inertia of the chain. This overtravel is quite unpredictable, varying according to the amount of heating the sheet has undergone in the machine, which makes it relatively easier to move along than when it is cold.
Thus, in order to achieve accurate indexing, it is necessary to drive such web advancing means at a much slower speed than lends itself to real economical production of the formed articles. Otherwise, inaccurate indexing and waste of material of the web matrix will be readily achieved.
The prior art has also attempted to utilize "hitch-style" feeds, wherein opposed edges of the web are gripped between upper and lower fingers which pull the web towards the discharge end of the apparatus. The fingers drop the web and are then returned to their initial position. As with the chain and pin mechanisms, this "hitch-style" feed has a high mass limiting the speed of the feed and the material gripped is prone to slippage as it is conveyed, reducing the indexing accuracy and promoting waste of material.
Accordingly, it has been proposed that the web stock be advanced by pulling it through the forming press by gripping the discharge end of the web after it leaves the press and intermittently pulling the web. However, when heat is applied to the web prior to its entry into or even while in the press, the web cannot be readily pulled through the equipment from the discharge end. This is because the heated web is softened and becomes substantially weakened. This does not allow a large tensile pulling force to be applied to any extent, without holding and feeding the web adjacent its weakened area; otherwise, the opposite edges of the web may become diagonally distorted, elongated, weakened, or wrinkled as it advances because of heating and cooling of the web and cutting of the same during the forming process, resulting in tearing or off-set feeding of the web.
Further, such web advancing apparatus has typically included a pair of tangential rollers forming a nip between which the opposed edges are threaded. This has tended to retard the speed and accuracy of indexing of the web because of slippage of the web in the roller nip.
These and related problems have been solved by use of the web advancing apparatus of the present invention in a substantially conventional thermoforming apparatus using pressure differential and mating die techniques to form articles in a forming press.